1. Field of the Invention
The present invention relates to signal control circuits and signal control apparatuses, and particularly to a signal control circuit having an external output terminal for a plurality of bits and a signal control apparatus for inputting and outputting data of a plurality of bits inside the apparatus.
2. Description of the Related Art
In general signal control circuits included in large-scale-integration (LSI) chips, noise resulting from a concurrent change in a plurality of bits of an output signal (for example, an output signal of eight bits is changed from “00000000” to “11111111”) output from the signal control circuit may cause one or more bits of the output signal to indicate a wrong value temporarily, which could lead to a malfunction. This type of malfunction can be avoided by reducing the absolute amount of noise through strengthening the power system or lowering the input or output speed. However, strengthening a power source and related circuits is costly, and lowering the input or output speed lowers the performance.
Some known methods monitor whether a concurrent change occurs and take measures against noise when a predetermined number of changes is exceeded (refer to Japanese Unexamined Patent Application Publication Nos. Hei-11-167530, 2004-318450, and Hei-7-84985). The method disclosed in Japanese Unexamined Patent Application Publication No. Hei-11-167530 delays a reference signal when the number of concurrent changes greater than or equal to a reference value occurs and validates data after the transient data error state due to noise ends. The methods disclosed in Japanese Unexamined Patent Application Publication Nos. 2004-318450 and Hei-7-84985 suppresses noise due to a concurrent change by dividing the data into parts of a certain bit width. When the data is divided, a write control signal is output for each corresponding bit in order to indicate whether the bit is valid or invalid.
The circuit configurations disclosed in the above three patent documents, however, adopt a method of making a concurrent judgment when data is transferred to an external circuit. The address and the transfer length are reported to the external circuit simultaneously with the data transfer, and the processing takes time.
Since there are various types of external circuits, a signal control method applicable to those various types of external circuits has been desired.